The present invention generally relates to a needle assembly for introducing a carrier gas into a sample vessel closed by a self-sealing membrane and, in particular, relates to an assembly useful as a sampling device in gas chromatography operating in accordance with the head space method.
A state of equilibrium in which the partial pressures of the sample components in the head space unambiguously depend on the composition of the sample is obtained in the head space above a sample liquid in a sample vessel closed by a membrane. With a sampling device operating in accordance with the head space method, the sample supplied to the separating column of a gas chromatograph is taken from this head space. For this purpose a needle is pierced through the self-sealing membrane. The needle is connected to the inlet of a dosing head of the gas chromatograph which is connected to a carrier gas conduit having a shutoff valve contained therein. Initially, carrier gas enters through the needle into the head space of the closed sample vessel with the shut-off valve open such that a carrier gas pressure builds up in the sample vessel. This does not change anything in the partial pressures of the sample components in the head space. When the shut-off valve is closed, the carrier gas pressure in the dosing head breaks down. Then a gas sample is pressed from the headspace into the closing head and the inlet of the separating column by the excess pressure in the sample vessel. After a well defined time, the shut-off valve is again opened whereby the dosing is terminated and the sample is transported into the separating column of the gas chromatograph (German Pat. No. 1 208 523).
To prevent the carrier gas from flowing unrestrictedly out of the needle after the needle has been removed from the sample vessel, it is known (German Pat. No. 1 284 660) to arrange the needle in a piston sealingly movable in a cylinder. The cylinder includes a restricting outlet and is closed on its end facing the sample vessel by a further self-sealing membrane. The cylinder is under the action of a compression spring which is supported on the piston and tends to push the cylinder over the needle. In the position of rest the needle with its opening being near the pointed end is in the interior of the cylinder.
In this known apparatus, the needle is stationary and always connected to the dosing head and the carrier gas conduit. The cylinder is longitudinally movably guided relative to the stationary needle. A sample vessel is placed with its self-sealing membrane into engagement with the end face of the cylinder closed by the further self-sealing membrane and is pushed upwards, the cylinder being pressed back thereby and the needle entering into the sample vessel through the two self-sealing membranes. In the position of rest, a rinsing flow flows through the needle, the rate of which flow is determined by the restriction of the outlet of the cylinder. The rinsing flow ensures that no vapors in the needle are carried over from one sample to the next.
By the German Offenlegungsschrift No. 28 15 023, a needle assembly is known in which the carrier gas is introduced into an apparatus-fixed housing and a needle is sealingly guided in sealing means in the housing for sliding movement between a position of rest and an operating position. The needle has a tip having a first lateral opening provided close to this tip. The needle has a second opening spaced from the first lateral opening. In the operating position of the needle, the second opening communicates with the chamber formed in the housing such that a carrier gas flows through the second opening into the needle and out of the first opening. When the sample vessel is pressed against the action of a return spring, the needle is longitudinally movably guided between a position of rest and an operating position. In the position of rest, the second opening is located in the chamber to which a restricted rinsing flow is supplied. This rinsing flow passes through the second lateral opening of the needle and emerges to atmosphere through the first lateral opening.
When the sample vessel is pressed on, the needle is pushed into its operating position directly or through a spring mechanism.
Conventional needle assemblies require the sample vessel to be lifted and pressed against the needle. Such mechanisms require considerable design expenditure, particularly when the sample vessels are arranged in a thermostatized turntable (German Pat. No. 1 297 904 or German Pat. No. 28 18 251).